1. Field of the Invention
The present invention relates to a method for evaluating a dependence of properties of a semiconductor substrate on a plane orientation (referred to as plane orientation dependence) and a semiconductor device manufactured using the method.
2. Description of the Prior Arts
The plane orientation is an important factor in physical properties of a crystalline semiconductor (e.g., silicon, etc.,) substrate including a rate of thermal oxidation, a rate of etching, electron mobility and the like. In particular, the plane orientation gives a great influence to the rate of thermal oxidation, i.e., a growth rate for a thermal oxidation film formed on a surface of a silicon substrate. The rate of thermal oxidation is the slowest on a surface of the silicon substrate having plane orientation (100), faster on a surface thereof having plane orientation (111) and still faster on a surface thereof having plane orientation (110). Therefore, the thickness of thermal oxide films is uneven unless they are formed on surfaces having the same plane orientations.
There is no problem in the case where a gate oxide film is formed only on a surface of a silicon wafer such as known MOSFET. However, it is important to obtain a gate oxide film of a uniform thickness in a device wherein the gate oxide film is three-dimensionally arranged, for example, in a trench-type vertical power MOSFET shown in FIG. 16. Therefore, in order to avoid an area where dielectric voltage is reduced, attempts have been made for matching all plane orientations of areas having formed thereon the gate oxide films by putting some thought into the arrangement of elements (chips) on the substrate (see Japanese Unexamined Patent Application No. HEI10-154810).
As understood from the above, it is important to determine the plane orientation of the substrate itself and evaluate the plane orientation dependence before manufacturing a device.
One of the conventional popular methods for evaluating the plane orientation dependence of a substrate is as follows. Wafers having different plane orientations are prepared by cutting out from silicon ingots having various plane orientations. These wafers are thermally oxidized actually and respectively, and the plane orientation dependence is evaluated by the rate of thermal oxidation.
However, this method entails a problem that the impurity concentration or the like is within specs but may be different in lots of ingots. This causes different rates of thermal oxidation, and therefore, the method is poor in precision for evaluating the plane orientation dependence.
Another conventional popular method for determining the plane orientation of a substrate is as follows. In manufacturing a semiconductor device, a reference pattern for mask alignment is formed in advance, and then, a mark pattern for a photomask alignment is matched to the reference pattern. The reference pattern is formed on a basis of an orientation flat or a notch (a V-shaped cut-out formed in an outer peripheral section of the silicon wafer) of the wafer. The orientation flat or notch is formed to provide a nominal reference plane by cutting a portion of the outer peripheral section of the wafer along a crystal orientation upon manufacturing the wafer.
The orientation flat formed on the silicon wafer having plane (100) provides a nominal plane (110). The edge, e.g., the corner, between plane (100) and orientation flat (110) of the silicon wafer has orientation <110>. The orientations generally exhibit precision with a margin of approximately plus or minus 2 degrees with respect to the true crystal orientation.
In other words, even if the orientation flat are accurately aligned in position with the mark pattern of the photomask, a deviation of 2° at the maximum may actually occur between the mark pattern for the photomask alignment and the true crystal orientation. Therefore, the plane orientation of the substrate cannot be determined accurately because the amount of deviation varies in every silicon wafer.
In order to eliminate such a positional deviation, Japanese Unexamined Patent Publication No. HEI 7(1995)-283117 discloses a photomask alignment method for determining a crystallographic reference orientation of a silicon wafer with high precision. This method does not use a cut line of the orientation flat as a reference for alignment of crystal orientation, but uses another means. The method will be explained with reference to FIG. 17.
Firstly, a silicon oxide film 72 is formed as an etching mask layer on a silicon wafer having plane (100). Then, two circular openings 70 and 71 are formed on the silicon oxide film 72 at the positions spaced from each other. The circular openings 70 and 71 are formed such that a straight line connecting the centers of the openings 70 and 71 is approximately parallel to the orientation flat showing nominal orientation <110> of the silicon wafer.
Thereafter, the resulting silicon wafer is isotropically etched with an alkali solution such as potassium hydroxide (KOH) solution. Thereby, the silicon wafer is etched from the circular openings 70 and 71 on the silicon oxide film 72, so that etch pits 74 and 75 each having a square pyramid shape with the circular opening 70 or 71 as the center are formed on the surface of the silicon wafer (FIG. 17(a)). These etch pits 74 and 75 are squares having four sides of orientation <110> on the surface of the silicon wafer.
Subsequently, a photomask 78 is prepared for obtaining a true orientation <110> of the silicon wafer. This photomask 78 is provided, as shown in FIG. 17(b), with a square window 76 and a rectangular window 77 arranged as follows to provide a mark line for mask alignment: The windows 76 and 77 are spaced from each other like the openings 70 and 71 and disposed in a parallel relation. The shorter sides of the windows 76 and 77 are smaller than the sides of the etch pits 74 and 75.
The photomask 78 is arranged and aligned on the silicon wafer having the etch pits 74 and 75 thereon. As shown in FIG. 17(c), the alignment is performed so that the sides of the etch pits 74 and 75 are parallel to the sides of the windows 76 and 77. This brings the mark line for the mask alignment directing the true orientation <110> of the silicon wafer indicated by reference numeral 80.
In this method, even if surfaces oriented in the orientation <110> are produced with respect to the circular openings 70 and 71 on the silicon oxide film 72 by isotropic etching using the alkali solution and thereby the photomask is aligned in the right position, this alignment is obtained only with respect to a positional alignment pattern, i.e., the circular openings 70 and 71. Such alignment is not always accurate with respect to a pattern for evaluating the plane orientation dependence or a pattern for an actual device in which plane orientations are required to be matched in consideration of potential deformation or warpage of silicon wafers which may vary every shot (depending upon silicon ingots). In other words, the photomask is aligned only in an indirect manner by this method.
The wafer obtained by the above-described process cannot be applied to a plane orientation evaluation pattern or an actual device. For, when a silicon wafer having plane (100) is etched in potassium hydroxide (KOH) solution, a surface thereof having plane (111) remains because dissolution rate is extremely slow in plane (111). Therefore, the etch pits formed on the silicon wafer have a V-shaped trench with an angle of about 55°. The etch pits cannot be applied to a trench-formation process in manufacture of the above-mentioned trench-type vertical power MOSFET, particularly.
In order to form on a silicon wafer a trench-shaped etch pit having vertical sidewalls, a special silicon wafer having a surface of orientation (110) is required to be used. It is difficult to apply such a special silicon wafer to a process flow established for a mainstream silicon wafer having plane (100).
Under such circumstances, there has been the demand for a technique for accurately and easily determining one or more plane orientations of a substrate and evaluating the dependence of arious properties of the substrate on the plane orientations by a single process.